1. Field of the Invention
The present invention relates to an ignition system for an internal combustion engine and, more particularly, an electronically controlled ignition system that includes a power switching element, an integrated control circuit (IC) and a protection circuit that protects the ignition system from extraneous electric surges.
2. Description of the Related Art
JP-A 10-274142 discloses an engine ignition system in which current supply to an ignition coil is controlled by a control integrated circuit (hereinafter referred to as the control IC) to control ignition timing. In such an ignition system, a zener diode is provided to protect the control IC from extraneous surges. The zener diode is connected to an input terminal of the control IC at its cathode and to a ground at its anode. If an extraneous surge comes to the ignition system, the zener diode bypasses the surge and prevents it from getting into the control IC.
Recently, there is a tendency to mount an ignition coil in each plug hole of an internal combustion engine. As shown in FIG. 7 of the present application, the ground terminal of an ignition system J1 is connected to an engine body J2, the ground terminal of an ECU (engine control unit) J3 for driving the ignition system J1 is connected to a vehicle body J6 (vehicle GND), the ground terminal of a starter J4 is connected to the engine body J4 (engine GND), and the negative terminal of a battery J5 is connected to the vehicle body J6 (vehicle GND). When starting current is supplied to the starter J4 from the battery J5 as indicated by curve A, a considerable voltage drop is generated between the engine body J2 (engine GND) and the vehicle body (vehicle GND) J6. In this case, the zener diode can not prevent a large amount of current flowing in the forward direction thereof caused by the voltage drop, which may result in failure of the control IC.
Further, the distance between the ignition coil J1 and the ECU J3 becomes longer, so that the length connecting wires connecting the ignition coil J1 and the ECU J3 increases. As a result, the stray capacitance of the connecting wires may cause fluctuation of the signals on the input terminal, which may cause a malfunction of the ignition system.